Organic film is high in flexibility as compared with glass, difficult to break, and lightweight. Recently, study has been performed with the aim of developing a flexible display using organic film as the substrate of a flat panel display.
Generally, resins used in organic film include polyester, polyamide, polyimide, polycarbonate, polyether sulfone, acrylic, and epoxy. Of these, polyimide resin is high in heat resistance, mechanical strength, abrasion resistance, dimensional stability, chemical resistance, and insulation capability, and accordingly in wide use in the electric/electronic industries.
For use as an alternative to the glass substrate in display elements, polyimide resin is required to have high transparency and low birefringence. These properties are necessary to obtain clear images.
To produce polyimide that is high in transparency and low in birefringence, a useful method is to use an alicyclic monomer as at least either of the two components: acid dianhydride and diamine (e.g. see patent document 1).
On the other hand, if a support substrate is coated with a polyimide precursor resin composition in the manufacturing process, followed by, in particular, heating it to cure the polyimide precursor to form polyimide resin film, there may occur problems including warp of the support substrate due to thermal stress and peeling of the polyimide resin film from the support substrate To avoid them, the polyimide is required have a coefficient of thermal expansion that is close to that of the support substrate.
Most polyimide products, however, have a high coefficient of linear thermal expansion (CTE) in the range from 50 to 100 ppm/K. In recent years, low-CTE polyimide materials have been proposed. These studies have reported that as essential requirements, the polyimide should have a straight, rigid backbone chain structure and the internal rotation should be in a restricted state (e.g. see non-patent document 1). Specifically, it is reported that wholly aromatic polyimide derived from an aromatic dianhydride and an aromatic diamine can work effectively. However, wholly aromatic polyimide gives an absorption band attributed to intramolecular or intermolecular charge transfer, which appears in the visible light wavelength range, resulting in polyimide resin film colored in yellow or brown.
As a solution to this problem, a polyimide produced from material having an alicyclic structure that can achieve transparency has been disclosed. It has been disclosed, for example, that polyimides produced from a combination of a trans-1,4-diaminocyclohexane, which is an alicyclic diamine of a rigid structure, and a rigid aromatic dianhydride have transparency and low CTE (e.g. see patent document 3). It has also been disclosed that polyester imides produced from an alicyclic dianhydride containing an ester group have high transparency and low birefringence (e.g. see patent document 2). It has been disclosed, furthermore, that copolymerized polyimides produced from 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, fluorene-containing aromatic dianhydride, and trans-1,4-diaminocyclohexane shows high transparency, low birefringence, and low CTE (e.g. see patent document 4).